1cupsemisweet chocolate chipsI used a combination of semisweet and 72% dark chocolate chunks
Mix together the flax and water and let sit until thickened, about 5-7 minutes.
Cream together butter and sugars until light and fluffy, about 5 minutes. Add flax and vanilla and continue creaming for 2 minutes.
Sift together the dry ingredients (flour, baking soda, salt, and ground espresso) and add to the mixing bowl. Mix until just combined, then fold in the chocolate chips.
Scoop the cookie dough (it'll be very soft!) onto a parchment-lined baking sheet. Cover lightly with plastic wrap and chill the dough for at least one hour. Once the hour is almost up, preheat oven to 350ºF.
Keep the cookies in the fridge until the oven is fully preheated. The cookies should be chilled before baking so that they stay tall and don't spread too much.
Sprinkle the cookie dough with a pinch of sea salt and a pinch of ground espresso. Bake the cookies 15-17 minutes until golden brown.
You can chill the dough for up to 72 hours before baking. The longer you let the dough chill, the more flavorful the cookies will be!